1. Field of the Invention
This invention relates to a process and an apparatus for producing substantially clean water and consolidated slugs of fuel from coal slurry.
2. Description of the Prior Art
In a coal mining operation, coal is removed from the mine and passed to a crushing plant where it is comminuted or crushed to facilitate removal of sulfur, ash and other impurities. During the crushing operation, the coal is washed with water to entrain particles of coal, sulfur and ash thereby greatly reducing the danger of fire, explosion and airborne coal dust hazards. Coal cleaning, which is required primarily to reduce the sulfur and ash content, has become an increasingly important factor in coal preparation in order to meet the tighter environmental standards. The following methods are currently employed to extract and upgrade the quality of the coal which methods result in the very fine-grained coal being lost to the washing and finally being disposed of as slurry refuse. These methods can also be employed to upgrade the quality of the slurry refuse.
The particles in the slurry can be classified by means of a hydrocyclone, a sizing device consisting of a conical-cylindrical apparatus which operates under a pressure of more than 5 psi. By means of rotational fluid-solid motion, particles in the slurry are separated according to their mass.
Another method of upgrading coal slurry is by concentration. Froth flotation is a complex physicochemical process which takes place in a slurry in which the surface of one or more minerals are made water-repellant and responsive to attachment to air bubbles. Beneficiation is accomplished when air bubbles are pumped into the slurry and coal-laden bubbles rise to the surface, leaving behind minerals which have not responded to the treatment. Flotation chemicals and reagents, called collectors and modifiers, attach themselves to the mineral surface through physical and chemical sorption.
Another method is mechanical dewatering in which water is removed by means of gravity or centifugal forces through screens or sedimentation. Sedimentation is used either for clarification or thickening. Thickening increases the concentration of solids in the slurry, whereas clarification is designed to produce a solid-free slurry. Using a centrifuge without screens, solids are segregated at the bottom toward the outside of the centrifuge and water is collected and decanted off from the center. The most common screening method used is vacuum filtration through a 40.times.60 mesh stainless steel screen. Air is sucked through the slurry and the screen resulting in a cake of solids and a filtrate which is drawn off.
A further method of dewatering coal slurry is by thermal drying. This can be accomplished by directly contacting the slurry with warm air, directly contacting the slurry with the heated shell of the dryer or heated particles, or by radiation from a hot surface to the slurry.
All the above-mentioned methods of slurry dewatering or extraction of fine coal are expensive and are being incorporated to various degrees in only the most recently built preparation plants. Those plants which have been in operation for some time do not have these facilities and it is either impractical or too expensive to install these new coal upgrading techniques. As a result, the fine particles of coal, clay, sulfur, iron and other impurities, which form a major part of the slurry refuse coming out of a substantial number of coal preparation plants currently under production, are still being pumped to settling or slurry ponds where the heavier particles settle out and some of the water may be returned to the plant for additional washing operations or otherwise disposed of.
The compositions of the many slurry ponds throughout the country vary widely depending on the composition of the coal being mined and the type of coal extraction and preparation operations. In fact, the composition in each individual slurry pond varies depending on particle sizes, location with respect to the inlet pipe and even in relation to such variables as the prevailing wind. Over the years many hundreds of acres of coal slurry has been collected in hundreds of ponds throughout the world. These slurry ponds are not only ugly blemishes on the countryside, but are hazardous to man and animal and detrimental to the environment. The vast quantities of water used to wash the coal become polluted by the coal particles and other associated mineral impurities washed from the coal resulting in large amounts of coal in the form of particles which cannot be reclaimed for use as a valuable fuel but are disposed of as waste.
Thus, a significant problem associated with coal processing is the dewatering and drying of the refuse products in slurry form. Fine coal handled or cleaned in slurry form in coal preparation plants must be dewatered to rend it suitable for conveying and blending, to decrease transportation costs, and to increase its heating value. Fine refuse dewatering is very difficult and expensive and is therefore not commonly used because it would represent a significant portion of the overall cost of coal washing.
Prior to the present invention, the coal particles in coal slurry could not be utilized for their fuel value without first removing most of the water since about 100 Btu/lb are lost for every 1 percent of water content in the coal and because the coal slurry is difficult to handle and convey. The centrifuging and heating methods presently used to remove the water are slow, expensive and inefficient. Once the water content has finally been reduced, the coal must then be compacted by a briquetting technique into a form which is easy to handle. However, in spite of these methods all of the polluted slurry ponds remain and more are constantly being built to meet the demands of the washing plants.
In U.S. Pat. Nos. 2,800,072, 3,276,594, 3,540,586, 3,762,560 and 3,900,403 filter presses are disclosed which product filter cakes and filtered liquids from slurries. U.S. Pat. No. 4,019,431 discloses a process for dewatering sludge by compression of sludge cakes between movable filter bands to force out the water. U.S. Pat. Nos. 436,044, 478,539, 504,098, 1,231,929, 1,344,261, 1,631,037 1,647,075, 2,076,315, 2,275,398, 2,623,432, 2,675,304, 2,937,080 and German Pat. No. 823,442 disclose various apparatus having cylinders and pistons for separating liquids from solids or liquid-solid mixtures and U.S. Pat. Nos. 2,331,126, 2,358,765, 2,697,979, 2,904,835, 3,055,290, 3,548,456 and 3,736,083 disclose various apparatus with cylinders and pistons for forming briquettes of particulate material. U.S. Pat. Nos. 4,049,390 and 4,049,392 disclose apparatus for extruding briquettes of a mixture of powdered coal and a binder. U.S. Pat. No. 3,288,293 discloses an apparatus for removing water from coal mud or peat. However, none of the prior art methods or apparatus indicated above disclose the production of substantially clean water and a useful fuel product from coal slurries by mechanical means.